1. Technical Field
The invention relates generally to roller cone drill bits. More particularly, the invention relates to a system for retaining a roller cone on a bearing journal of the drill bit.
2. Background Art
Roller cone drill bits are generally made of a plurality of legs that are welded together to form a unitary structure. FIG. 1 shows an example of a drill bit leg 2 which defines a shirttail portion 4 and a cantilevered journal 6. The journal 6 forms a main bearing surface 8 for a roller cone 10. The roller cone 10 has cutting elements 12 which are adapted to deform earth formation as the drill bit leg 2 is rotated within a borehole. The roller cone 10 is retained on the journal 6 by a ball lock system which includes balls 16 that are retained between ball races 18 and 20 on the journal 6 and the roller cone 10, respectively. To assemble and lock the roller cone 10 to the journal 6, the balls 16 are inserted between the journal 6 and the roller cone 10 through a ball hole 22 which is drilled through the shirttail portion 4 and the journal 6. The ball hole 22 intersects the top dead center 24 of the ball race 18. The balls 16 are retained between the journal 6 and the roller cone 10 by welding a ball plug 25 in the shirttail side of the ball hole 22.
The drill bit leg thus described retains a roller cone on a journal using a ball lock system. However, there are other methods of retaining a roller cone on a journal, for example, segmented cone retention rings disposed in a slot on the journal and subsequently threadedly locked to the roller cone using a hole to gain access to a device which prevents rotation of the rings. Generally, any cone retention system that includes a hole penetrating a ball race or other slot on the journal will induce localized stresses in the ball race or slot. In particular, when the hole intersects a high stress region on the ball race, localized stresses which develop around the intersection of the ball race with the ball hole during operation of the drill bit may be sufficient to initiate cracks in the ball race and, possibly, break the journal. It would, however, be desirable to access the ball race or other slot on the journal without initiating cracks in the journal.